online TempoEstimationProcessor

[superbock]

This PR makes TempoEstimationProcessor online capable.

TempoEstimationProcessor.interval_histogram is refactored to use a dedicated histogram processor which is used to build the beat interval histogram. Different TempoHistogramProcessor classes provide the needed functionality for both online and offline mode, namely CombFilter, ACF and DBN. With the work done in #286 this change was unavoidable, otherwise TempoEstimationProcessor would have had too many options and parameters.

TODOs before this PR can be merged:

• Make these histogram processors capable of processing the activations as a whole (i.e. single --online mode) or frame by frame (i.e. online mode).
• Add option to set length of history to be considered when "integrating" the tempo. E.g. in offline mode comb filter histogram building sums all maxima of a musical piece. In online mode simply summing all values is not practicable, otherwise tempo changes cannot be tracked.
• Decide if we want to keep interval_histogram and interval_histogram_online methods or refactor the histogram stuff into dedicated classes (with the usual process_offline and process_online methods).
• Decide the output frequency in online mode, i.e. when to output the tempi. Output for every frame or whenever the tempo changes? Maybe this should be controllable via a parameter. (EDIT: outputting it at every frame now, could be changed easily if needed)
• Add activation smoothing for online mode? (EDIT: see Add activation smoothing for online mode #343)
• Refactor the visualisation stuff as mixins (EDIT: see refactor visualisation stuff #344)